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| 200P London, UK Pharmacology 2017 |
The risk of QTc prolongation in hospitalized patients on Methadone maintenance therapy.
Introduction: Methadone is a synthetic opioid. It is known to cause extended ventricular action potentials, reflected as a QT prolongation on the electrocardiogram (ECG) which increases the risk of re-entrant tachycardias such as Torsades de Pointes (TdP). Methadone is subject to pharmacokinetic and pharmacodynamic drug-drug interactions. Physiological and pathological factors also predispose a patient to QTc prolongation. The aim of this study is to identify inpatients at risk of QTc prolongation because of Methadone treatment and compare their monitoring with current hospital guidelines.
Method: This prospective study took place over two weeks. Patient demographics and background medical diagnoses were available in the patient’s medical chart and electronic patient record. The patient’s medical chart was reviewed for ECGs, electrolyte levels, liver function tests, eGFR and echocardiograms. A prolonged QTc was defined as >450ms for males and >470 for females. The patient’s drug chart was reviewed for high risk medications and the patient’s current Methadone dose recorded. Data was recorded and analysed using Microsoft Excel.
Results: The study included 22 patients from St James’s Hospital. The mean age was 41 years. The mean Methadone dose was 66ml. None had severe liver disease, documented heart disease or structural cardiac abnormalities. One had severe kidney disease (eGFR<30). 6(27.3%) were on drugs with pharmacokinetic interactions with Methadone. 3(13.6%) were on medications with pharmacodynamic interactions with Methadone. 3 were on combinations of drugs that had both interactions with Methadone. 7(31.8%) did not have an ECG in their chart. The mean QTc was 432ms. 9(60%) had prolonged QTc. 3 had QTc above 500ms. 14(63.6%) had incomplete electrolyte readings. 7(31.8%) had abnormal electrolyte readings. Electrolyte imbalances like hypomagnesemia increases the risk of developing TdP. 10(45.5%) did not have recorded magnesium levels. 5 had hypomagnesemia. 1 of the patients with QTc>500 had hypomagnesemia and another did not have Magnesium levels taken. 2 patients on Methadone doses>100ml had no ECG in chart.
Conclusion: This study indicated a deficiency in the monitoring of patients on Methadone especially given the additional risk factors that potentiate Methadone’s arrhythmogenic effect. St James’s Hospital recommends considering an ECG for patients before prescribing Methadone and after they are started on any drugs that increases the risk of QTc prolongation. In many cases, no ECG was evident in the patient chart. An ECG on admission provides a baseline measurement and is recommended in guidelines suggested by the University of Rochester Medical Centre.[1]
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